1. Field of the Invention
The invention relates generally to respirator filters and in particular to a respirator filters including a pleated filter layer.
2. Prior Art
Filtration face masks or respirators are used in a wide variety of applications when it is desired to protect a human's respiratory system from particulates or noxious gases. Recent changes in the regulations governing respirator requirements (42 CFR 84) mandate that filter media must have efficiencies of 95% or higher when tested against either sodium chloride or DOP aerosol, at a prescribed concentration, flow rate and particle size distribution. When the filter surface area is small, it is difficult to meet this requirement without exceeding the maximum pressure drop requirements of the standard. Typically, glass fiber media are used against DOP aerosols to achieve the desired higher efficiencies. The pressure drops across these glass fiber media are high enough that pleating must be employed to increase the filter surface area in order to minimize the pressure drop. These filters are typically potted in cartridge shells in order to provide a means for sealing the filter in a holder. The resulting filter is expensive to manufacture.
Electrostatic media, comprised of polypropylene melt blown fibers, are effective against sodium chloride aerosols and have been further developed to be efficient against DOP aerosols. The advantage of the polypropylene media is that it can be welded with other filter layers, pre-filter layers or protective scrims to create filter disks which can be inserted into clips and covers for sealing against an adapter or cartridge. No secondary potting or adhesive compounds are needed. For applications where the surface area is low, however, the filter may still have a pressure drop which is higher than desirable. FIG. 1A is a cross sectional view of a conventional flat filter 15. The filter is made up of a flat filter layer 16 made from a fibrous, thermoplastic material (such as an acrylic, polyester, nylon, polypropylene). Adjacent to the first filter layer 16 is flat filter layer 17 made from melt-blown, thermoplastic material (such as polypropylene). A protective layer 18 (e.g. spun bonded polypropylene) is included on the outside of filter layer 17. A drawback to the filter of FIG. 1A is that there is a small filtering surface area and thus a high pressure drop across the filter. This makes the filter difficult to breathe through.
It is well known in the industry that pleating can be used to increase the amount of filtering surface within a given area. By pleating filter media such as that which is composed of polypropylene melt blown fibers, the effective filtration surface can be increased and the pressure drop reduced. FIG. 1B is a cross-sectional view of a filter element 1 as described in U.S. Pat. No. 5,736,041. The filter element 1 includes a pleated filter layer 2 covered on both sides by cover layers 3a and 3b. Cover layers 3a and 3b protect the pleated filter media and serve as spacers to keep the folds of the pleated filter layer 2 separate. Layers 2, 3a and 3b are enclosed in an envelope 5. A drawback to this filter is that because the filtration efficiency is dependent solely on the pleated media, the height of the pleats must be quite large, on the order of 20 mm, to achieve high efficiencies. This makes the filter element thick and bulky and does not allow for designs which are low profile.